The invention discloses a system and method for softening treatment of high-density polyethylene paper by a flash evaporation method. The method is characterized in that a non-woven fabric piece is softened by a three-step method sequentially comprising breaking by button beaters, cloth crumpling and breaking by the button beaters. According to the system and the method, the button beaters are improved and can execute breaking in the reverse cloth feeding direction. The non-layered piece with designated softness can be made at a commercially feasible yield. Compared with an existing two-step method in which a crumpling device is arranged in front of or behind 'button beaters', the same designated softness can be achieved, the product yield is higher, the softness in the longitudinal direction and the softness in the transverse direction are basically the same, and the drapability of the product is greatly improved.